German Pat. No. 1,778,457 teaches a method for the manufacture of multi-layer shaped parts with a foamed core and an unfoamed thermoplastic shell. A mold is first injected with a plug of a first charge of unfoamable thermoplastic material which does not fill the mold. Then, prior to the solidification of the mid-section of this first charge plug, there is injected into this mid-section a second charge of thermoplastic material which contains a foaming agent. The material of the second charge presses the material of the first charge in every direction against the mold, so that this becomes fully filled. In this known method the second charge can be introduced only if the feeding of the first charge is interrupted or completely ended. This frequently results in undesirable markings at the surface of the finished shaped parts. Furthermore, particularly on shaped parts of a complex form, the first injected charge is sometimes not uniformly pressed against the walls of the mold by the subsequent charge that contains the foaming agent, but is so widely dispersed that it ruptures. This causes an interruption on the smooth surface of the shell.
German Pat. No. 2,241,002 teaches a further method for the injection molding of plastic parts with smooth surfaces and porous cores, wherein, subsequent to the injecting of the part of the plastic that forms the smooth surface, there is injected a plastic that contains a foaming agent simultaneously with additional plastic that forms the smooth surface. The plastic that contains the foaming agent flows into the mold through a central nozzle, while the plastic that forms the smooth surface arrives in the mold through a ring nozzle which surrounds the central nozzle. In this method there arrives (during the simultaneous injection phase of the two different plastics) only a small amount of the plastic that forms the smooth surface at the mold wall lying opposite to the sprue of the mold. This means that on this side of the finished shaped part (the so-called "visual" side) the shell of the plastic that forms the smooth surface is thin and sometimes is even torn. On the side of the finished shaped part adjacent to the sprue, the shell is, however, unnecessarily thick. In this method it is not possible to influence the thickness of the smooth-surfaced shell, e.g. through increase of the amount of plastic which forms the smooth surface, at the side that lies opposite to the sprue, in an economically justifiable manner. These and other difficulties experienced with the prior art devices have been obviated in a novel manner by the present invention.
It is, therefore, an outstanding object of the invention to provide a method and apparatus for the intermittent manufacture of multi-layer shaped parts of thermoplastics with a foamed core and an unfoamed shell, wherein it is assured that without an increase of the amount of plastic which forms the unfoamed shell, at least the surface opposite to the sprue (that is, the visual surface of the finished shaped part) displays a sufficiently thick shell and, above all, a compact shell with a smooth surface.
With these and other objects in view, as will be apparent to those skilled in the art, the invention resides in the combination of parts set forth in the specification and covered by the claims appended hereto.